“He eats only wet food, drinks from a plastic cup like a baby’s bottle and is spoon-fed his meds.

“It would be a tragic thing to see if it was someone other than him.

“Nurses visit three times daily and he’ll be lucky to see the end of the year.”

Sinclair raped and strangled Helen and Christine, both 17, after a night out at the World’s End pub in 1977.

In November 2014, the case saw him become the first person in Scotland to be re-tried for the same crime after a previous acquittal.

Sinclair was sentenced to a minimum 37 years in jail. It’s the longest term in Scottish legal history – and the same number of years the girls’ families waited to get justice.

It means he’ll not become eligible for parole until he is 106.

Sinclair challenged the sentence but his appeal was rejected last year.

Our prison source added: “Sinclair is now skin and bones. He can’t walk from his bed to the toilet unaided.

“He used to have a walking frame but now has a wheelchair.

First victim Catherine Reehill

“He knows when he’s being spoken to, and it’s believed he knows what’s being asked of him, but that’s about it – he’s incapable of doing most things.

“Sinclair has an alarm on his wrist which he can activate if he falls over or wants assistance.

“People do not relish having to attend to him but they have a duty of care. He still has an evil look in his eyes.”

The serial killer’s convictions, which also include theft, rape and culpable homicide, date back to 1959. In 1961, aged 16, he admitted sexually assaulting and strangling his young neighbour Catherine.

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He was sentenced to 10 years in prison for culpable homicide. A psychiatrist’s report warned Sinclair would remain a “very dangerous sexual case”. But he served just six years before being set free to murder again.

In 1978, Sinclair strangled 17-year-old Mary. He wasn’t convicted of her murder until 2001, when a cold-case review found DNA evidence linking him to her body.

Murder victim Mary Gallacher(Image: Daily Record)

In 2009, an FBI expert said he feared Sinclair had killed at least eight women.

Sinclair also emerged last year as a suspect in the 1977 murder of Frances.

Thomas Ross Young was jailed for the crime but always protested his innocence. He died in 2014 while waiting to hear if his case could be appealed.